This OVA was created as a stand alone adventure, so if you're looking for a manga based movie then forget it. The only things that are the same are names of people and locations. The biggest difference in this movie is character relations. People who are friends or would become friends in the manga and TV series are enimies all the way through the movie. I personally like the movie because it gives a fresh perspective on a proven title, but don't go watching this movie expecting certain people to be the same as in the manga and TV series or you'll dissapoint yourself.

Japan has an affection and an affinity for making movies and TV shows about giant things battling each other. The kaiju genre of film is one of Japan's most well known exports with Godzilla being a recognisable icon no matter where in the world you are. And not just kaiju films, but also things like Gundam or Power Rangers feature giant robots that fight to protect a city (which they usually end up trashing).
The Rayearth OVA is that kind of anime. It's basically what would happen if you took Power Rangers made the rangers cute anime girls and added a whole bunch random fantasy elements
to it. It's an odd mix to say the least.

The best part of this OVA is definitely its presentation. The anime looks great with its character and monster designs and overall aesthetic. It knows how to use colour, portraying the world as bleak and dark yet maintaining splashes of colour for the characters you're supposed to follow so they pop out. There drawings are detailed and nowhere in the animation does it feel like they slacked with all of the explosions, buildings collapsing and monsters firing giant laser beams looking impressive.
There are faults here too however. The backgrounds are rather indistinct, the entire anime takes place in Tokyo, but most of the time you can't really tell where everything is happening. This could be attributed to the OVAs overall dark colour palette and to the fact that most of the city gets reduced to indistinct piles of rubble anyway.

Speaking of piles of rubble, let's talk story. Rayearth is about three girls Hikaru, Umi and Fuu who get chosen by deities to be the protectors of our world the eponymous Rayearth. When the world gets attacked by the forces of a parallel world called Cephiro it's up to these three girls to pass their deity's test and become giant mech/monster hybrids and destroy the city... I-I mean save the world.
There's a few problems with how Rayearth executes this premise, and I will preface my listing my grievances with the saying that I have not seen the TV show or read the manga so I went into this blind.

Firstly, there's no sense of weight to the plot. Events happen in the show and their strung along in a nice sequence, but it feels like most of the scenes are standalone. There's no impact and at times it feels like things are happening just because. This, I feel, can be mainly attributed to how none of the characters in the show get any development. There is some attempted arc for the three girls but it comes off as forced and artificial, it's less of an arc and more of a straight line. One moment they're unsure, scared and in despair and the next BOOM they're a giant fire-breathing lion vanquishing their enemy with ease!
Secondly, the show is just plain confusing. The total running time of the three episodes is somewhere around 120 minutes yet for most of that time I had no clue why things were happening as they were on screen. Sure, the anime tries to explain itself somehow, but in all of its attempts it just somehow becomes more confusing. The motivation of the main antagonist is foggy at best, and to top it all of it ends on a very weird note and quite abruptly at that. It feels like they had a story written for a much longer series, but had to cut a lot out to fit three episodes. Remember however, that this is a completely original story written specifically for these OVAs that has nothing to do with the TV show or manga so how and why is all of this so confusing?

And the confusion doesn't end there either. There's more! Besides having a plot that's reminiscent of a maze with no exit it also can't get it's main theme straight. An important and often mentioned point in the show is the friendship between the three main girls yet for most of the show the girls get separated and only towards the end do we see some semblance of teamwork from them. The rest of the time we only get periodic reminders on how they're best friends and how important that is. Not feeling that butchering a story about friendship is enough, on the Cephiro side of things they try to shoehorn in a forbidden love story wrapped in a tale about nature. Basically, the fairy queen that governs nature and a high priest love each other, but that's a no-no so the world becomes barren. What's the message they're trying to send here? Don't fuck with nature? I don't get it.

Rayearth the OVA is a somewhat pretty looking and utterly confusing OVA that was probably made for people who've already seen the TV show and have some sort of connection and pre-existing knowledge about the world and characters. Coming in as an outsider who has no idea about any of this it comes off as a very confusing and confused anime that will ultimately leave you scratching your head and wondering why you wasted your time on it.
I did not enjoy my time with his OVA at all and I wish I had spent it somewhere else instead and I suggest you do too unless you have some knowledge about the world or like seeing giant monsters battling it out and destroying a city in the process.